Self assembled quantum dots, or Stranski Krastanow dots, hereinafter interchangeably referred to as SK dots or simply quantum dots, are used as efficient one dimensional quantum wells in semiconductor devices. Such quantum dots have been realized in a multitude of semiconductor material systems, such as InAs/GaAs, InAs/InP, InP/GaAs, InGaN/GaN, InN/GaN, GaN/AlGaN etc., either on planar surfaces or grown along edges.
Semiconductor quantum dots can be formed during epitaxial growth of a semiconductor layer on a substrate that is lattice mismatched to the semiconductor layer. In this growth process the dot formation is encouraged by minimizing the energy associated with crystal strain. The size of the dots formed is determined by fundamental parameters such as the surface energy of the dot facets and the interface between the dot and the substrate together with the accommodated crystal strain. In most applications a homogeneous size distribution is important, since slight variation in size will alter the relative position of the energy states of valence and conduction band. In several growth techniques the size of the dots can be varied within a limited range while retaining a fairly homogeneous size distribution by controlling growth conditions. However, for many applications the dot sizes vary too much.
The effect of quantum dots may be utilized in e.g. light emitting diodes (LED), transistors, solar cells, etc. For example, the quantum dots can be used to adjust the colour of the light emitted from a LED device. In general, larger quantum dots yield a redder light (longer wavelength) and smaller quantum dots yield a bluer light (shorter wavelength). Lightly strained quantum dots yield a redder light and more highly strained quantum dots yield a bluer light. Consequently it is a challenge to obtain a certain wavelength of the emitted light if there is a large lattice mismatch giving high strain levels. Furthermore, the wavelength can be altered by changes in the composition of the dots. Thus, the effect of the quantum dots relies on dot size, dot composition and strain.
In particular, growth of III-Nitride (III-N) semiconductor quantum dots in nitride based III-V materials, such as GaN, which are of special interest for LED applications, while retaining a sufficiently homogeneous size distribution has been shown to be hard to achieve. Investigations utilizing different carrier gas mixtures in order to manipulate column three material diffusion lengths and desorption conditions have shown to be insufficient for the fabrication of homogenously sized quantum dots. This indicates that it is the substrate surface quality that causes variation in local growth conditions for the quantum dots. This can for example be due to the high defect density and surface roughness of the low quality substrate, which prevent homogeneous kinetic conditions for the source materials on the substrate surface. Such local variations will not only degrade the size homogeneity but also the homogeneity of composition of ternary and quaternary quantum dots and owing to the variations in size and composition of individual dots the strain conditions of individual dots will vary.
Nitride based III-V materials, such as GaN, which are of special interest for LED applications have a high defect density due to the lack of compatible substrates. For GaN based devices SiC, Al2O3, and Si are most commonly used. These materials are lattice mismatched with respect to GaN, which causes a high defect density in the GaN. Also, they suffer from a high thermal expansion mismatch with respect to GaN. Moreover, SiC and Al2O3 are expensive and not yet commercially available in large wafer sizes.